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1.
The eastern part of the Cordillera Occidental of Ecuador comprises thick buoyant oceanic plateaus associated with island-arc tholeiites and subduction-related calc-alkaline series, accreted to the Ecuadorian Continental Margin from Late Cretaceous to Eocene times. One of these plateau sequences, the Guaranda Oceanic Plateau is considered as remnant of the Caribbean–Colombian Oceanic Province (CCOP) accreted to the Ecuadorian Margin in the Maastrichtien.Samples studied in this paper were taken from four cross-sections through two arc-sequences in the northern part of the Cordillera Occidental of Ecuador, dated as (Río Cala) or ascribed to (Macuchi) the Late Cretaceous and one arc-like sequence in the Chogòn-Colonche Cordillera (Las Orquídeas). These three island-arcs can clearly be identified and rest conformably on the CCOP.In all four localities, basalts with abundant large clinopyroxene phenocrysts can be found, mimicking a picritic or ankaramitic facies. This mineralogical particularity, although not uncommon in island arc lavas, hints at a contribution of the CCOP in the genesis of these island arc rocks.The complete petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature (MgO values range from 6 to 11 wt.%). Studied samples display marked Nb, Ta and Ti negative anomalies relative to the adjacent elements in the spidergrams characteristic of subduction-related magmatism. These rocks are LREE-enriched and their clinopyroxenes show a tholeiitic affinity (FeOT–TiO2 enrichment and CaO depletion from core to rim within a single crystal).The four sampled cross-sections through the island-arc sequences display homogeneous initial Nd, and Pb isotope ratios that suggest a unique mantellic source for these rocks resulting from the mixing of three components: an East-Pacific MORB end-member, an enriched pelagic sediment component, and a HIMU component carried by the CCOP. Indeed, the ankaramite and Mg-basalt sequences that form part of the Caribbean-Colombian Oceanic Plateau are radiogenically enriched in 206Pb/204Pb and 207Pb/204Pb and contain a HIMU component similar to that observed in the Gorgona basalts and Galápagos lavas. The subduction zone that generated the Late Cretaceous arcs occurred far from the continental margin, in an oceanic environment. This implies that no terrigenous detrital sediments interacted with the source at this period. Thus, the enriched component can only result from the melting of subducted pelagic sediments.We have thus defined the East-Pacific MORB, enriched (cherts, pelagic sediments) and HIMU components in an attempt to constrain and model the genesis of the studied island-arc magmatism, using a compilation of carefully selected isotopic data from literature according to rock age and paleogeographic location at the time of arc edification.Tripolar mixing models reveal that proportions of 12–15 wt.% of the HIMU component, 7–15 wt.% of the pelagic sediment end-member and 70–75 wt.% of an East-pacific MORB end-member are needed to explain the measured isotope ratios. These surprisingly high proportions of the HIMU/CCOP component could be explained by the young age of the oceanic plateau (5–15 Ma) during the Late Cretaceous arc emplacement. The CCOP, basement of these arc sequences, was probably still hot and easily assimilated at the island-arc lava source.  相似文献   

2.
The eastern part of the Cordillera Occidental of Ecuador comprises thick buoyant oceanic plateaus associated with island-arc tholeiites and subduction-related calc-alkaline series, accreted to the Ecuadorian Continental Margin from Late Cretaceous to Eocene times. One of these plateau sequences, the Guaranda Oceanic Plateau is considered as remnant of the Caribbean–Colombian Oceanic Province (CCOP) accreted to the Ecuadorian Margin in the Maastrichtien.Samples studied in this paper were taken from four cross-sections through two arc-sequences in the northern part of the Cordillera Occidental of Ecuador, dated as (Río Cala) or ascribed to (Macuchi) the Late Cretaceous and one arc-like sequence in the Chogòn-Colonche Cordillera (Las Orquídeas). These three island-arcs can clearly be identified and rest conformably on the CCOP.In all four localities, basalts with abundant large clinopyroxene phenocrysts can be found, mimicking a picritic or ankaramitic facies. This mineralogical particularity, although not uncommon in island arc lavas, hints at a contribution of the CCOP in the genesis of these island arc rocks.The complete petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature (MgO values range from 6 to 11 wt.%). Studied samples display marked Nb, Ta and Ti negative anomalies relative to the adjacent elements in the spidergrams characteristic of subduction-related magmatism. These rocks are LREE-enriched and their clinopyroxenes show a tholeiitic affinity (FeOT–TiO2 enrichment and CaO depletion from core to rim within a single crystal).The four sampled cross-sections through the island-arc sequences display homogeneous initial Nd, and Pb isotope ratios that suggest a unique mantellic source for these rocks resulting from the mixing of three components: an East-Pacific MORB end-member, an enriched pelagic sediment component, and a HIMU component carried by the CCOP. Indeed, the ankaramite and Mg-basalt sequences that form part of the Caribbean-Colombian Oceanic Plateau are radiogenically enriched in 206Pb/204Pb and 207Pb/204Pb and contain a HIMU component similar to that observed in the Gorgona basalts and Galápagos lavas. The subduction zone that generated the Late Cretaceous arcs occurred far from the continental margin, in an oceanic environment. This implies that no terrigenous detrital sediments interacted with the source at this period. Thus, the enriched component can only result from the melting of subducted pelagic sediments.We have thus defined the East-Pacific MORB, enriched (cherts, pelagic sediments) and HIMU components in an attempt to constrain and model the genesis of the studied island-arc magmatism, using a compilation of carefully selected isotopic data from literature according to rock age and paleogeographic location at the time of arc edification.Tripolar mixing models reveal that proportions of 12–15 wt.% of the HIMU component, 7–15 wt.% of the pelagic sediment end-member and 70–75 wt.% of an East-pacific MORB end-member are needed to explain the measured isotope ratios. These surprisingly high proportions of the HIMU/CCOP component could be explained by the young age of the oceanic plateau (5–15 Ma) during the Late Cretaceous arc emplacement. The CCOP, basement of these arc sequences, was probably still hot and easily assimilated at the island-arc lava source.  相似文献   

3.
According to palinspastic reconstructions, the Neo-Tethys opening took place during the Permian between the Cimmerian fragments in the north and the Indo-Arabian margin in the south. Igneous remnants of this opening are exposed in Oman within either the Hawasina nappes or the para-autochtonous Arabian platform exposed in the Saih Hatat tectonic window. They consist predominantly of pillowed basaltic flows among which three groups have been distinguished. Group 1 is tholeiitic and characterized by low TiO2 and incompatible trace element contents, and a large range of Ndi values. Group 1 basalts are associated with distal sediments and plot near the boundary of or within the MORB field in the Pb–Pb correlation diagrams and between the MORB and Bulk Silica Earth (BSE) fields in Ndi–(206Pb/204Pb)i diagram. Group 2 basalts are alkaline and differ from Group 1 ones by their higher TiO2, La and Nb contents, and lower and more homogeneous Ndi values (+3 to +5). Group 2 volcanics are similar to alkali basalts from oceanic islands and share with Group 1 similar initial Pb ratios. Group 3 consists of tholeiitic and alkali basalts which are interbedded either with carbonate-platform sediments from the Saih Hatat window or with distal sediments from the Hawasina Nappes. This group differs from Groups 1 and 2 by its low to negative Ndi (+1.6 to −2). Group 1 likely derived from the mixing of depleted and enriched sources while Group 2 derived exclusively from an enriched source. There is no indication that continental crust was involved in the genesis of both Groups 1 and 2. In contrast, the low to negative Ndi values of Group 3 suggest that the magmas of this group were contaminated by the Arabian continental crust during their ascent. The geochemical features of the Middle Permian plume-related basalts suggest thus that the basement of the Hawasina basin was not genuine oceanic crust but either the thinned Arabian rifted continental margin or the continent–ocean transition zone of the Neo-Tethys.  相似文献   

4.
The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks underwent at least two stages of post-magmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. These alteration processes resulted in the mobilization of many major and trace elements. The trace element characteristics of the least altered rocks are consistent with a supra-subduction zone geodynamic setting and shallow mantle sources. On the basis of geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of a lower layer of leucogabbros and anorthosites, and an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and sills, and dunitic to wehrlitic sills.

The Sm–Nd and U–Pb isotope systems have been disturbed in strongly altered actinolite schists. In addition, the U–Pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration. Gabbros and diorites have the least disturbed Pb isotopic compositions. In contrast, the Sm–Nd isotope system appears to have remained relatively undisturbed in picrites, pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (εNd = + 4.23 to + 4.97) than pillow lavas, gabbros, and diorites (εNd = + 0.30 to + 3.04), consistent with a variably depleted, heterogeneous mantle source.

In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (> 90%) of Ca-rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas. The anorthositic cumulates have significantly higher initial εNd (+ 4.8 to + 6.0) values than the surrounding gabbroic matrix (+ 2.3 to + 2.8), consistent with different mantle sources for the two rock types.  相似文献   


5.
Basaltic lavas of Turonian to Coniacian age belonging to the Bath–Dunrobin Formation occur with intercalated island arc tuffs in the south of the Blue Mountain inlier, have been interpreted as being derived from the Caribbean oceanic plateau. This study presents new major and trace element and Sr–Pb–Nd–Hf isotopic data for these igneous rocks. The Jamaican rocks are altered by tropical weathering, hydrothermal and metamorphic processes, which have mobilised many of their elements (e.g. K and Ba). Consequently, the basalts and dacitic tuffs have been classified by using immobile trace elements. The trace element and Hf(i)–Nd(i) geochemistry suggests that the basaltic lavas are derived from a chemically similar source region by variable degrees of partial melting. The Caribbean plateau basalts lie on a mixing line between a depleted plume component and HIMU in Nd–Hf isotopic space. The Pb isotope data also demonstrate that the Jamaican plateau lavas are composed of a larger HIMU component than the other plateau lavas within the Caribbean region. The intercalated island arc tuffs are the first to be found in any oceanic plateau succession in the Caribbean and imply that the Caribbean oceanic plateau at  90 Ma was relatively close to the subduction zone along South America and the Great Arc of the Antilles.  相似文献   

6.
The Ungava orogen of northern Québec is one of the best preserved Proterozoic mobile belts of the world, recording > 200 Ma of plate divergence and convergence. Voluminous magmatism associated with rifting of the Superior Province basement ≈2.04 Ga resulted in the development of a volcanic rift margin sequence and an ocean basin. Four distinct mafic magma suites were erupted: (1) continental basalts (Eskimo Formation, western and central Povungnituk Group) with moderate to high Zr/Nb and negative Nb anomalies which have interacted with the continental crust (εNd(2.0 Ga)) from −7.4); (2) mafic lavas from the Flaherty Formation, eastern Povungnituk Group and some Watts Group lavas associated with passive margin rifting, having slightly enriched isotopic signatures (εNd(2.0 Ga) = +2.7 to +4.4) compared to the contemporaneous depleted mantle, high (Nb/Y)n and low Zr/Nb ratios (≈4.4 and ≈8.9, respectively); (3) a highly alkaline OIB-like suite (εNd(2.0 Ga) = +2.3 to +3.2, (Nb/Y)n> 12) within the Povungnituk Group composed of nephelinites, basanites and phonolites; and (4) depleted Mg-rich basalts and komatiitic basalts (εNd(2.0 Ga) ≈ + 4.5 to + 5.5) with trace-element characteristics of N-MORB, but with higher Fe and lower Al than primitive MORB (Chukotat Group, Ottawa Islands and some Watts Group samples). The ocean basin into which these lavas were erupted was subsequently destroyed during subduction between ≈1.90 and ≈1.83 Ga, resulting in the development a magmatic arc (Narsajuaq terrane and Parent Group).

The Ungava magmas provide a unique window into the mantle at 2.0 Ga. The chemical and isotopic similarity of these Proterozoic magmas to modern-day magmas provides strong evidence that the interplay between depleted mantle, OIB mantle and sub-continental mantle during the Proterozoic was comparable to that of the modern Earth.  相似文献   


7.
We present new regional petrologic, geochemical, Sr–Nd isotopic, and U–Pb geochronological data on the Turonian–Campanian mafic igneous rocks of Central Hispaniola that provide important clues on the development of the Caribbean island-arc. Central Hispaniola is made up of three main tectonic blocks—Jicomé, Jarabacoa and Bonao—that include four broad geochemical groups of Late Cretaceous mafic igneous rocks: group I, tholeiitic to calc-alkaline basalts and andesites; group II, low-Ti high-Mg andesites and basalts; group III, tholeiitic basalts and gabbros/dolerites; and group IV, tholeiitic to transitional and alkalic basalts. These igneous rocks show significant differences in time and space, from arc-like to non-arc-like characteristics, suggesting that they were derived from different mantle sources. We interpret these groups as the record of Caribbean arc-rifting and back-arc basin development in the Late Cretaceous. The> 90 Ma group I volcanic rocks and associated cumulate complexes preserved in the Jicomé and Jarabacoa blocks represent the Albian to Cenomanian Caribbean island-arc material. The arc rift stage magmatism in these blocks took place during the deposition of the Restauración Formation from the Turonian–Coniacian transition (~ 90 Ma) to Santonian/Lower Campanian, particularly in its lower part with extrusion at 90–88 Ma of group II low-Ti, high-Mg andesites/basalts. During this time or slightly afterwards adakitic rhyolites erupted in the Jarabacoa block. Group III tholeiitic lavas represent the initiation of Coniacian–Lower Campanian back-arc spreading. In the Bonao block, this stage is represented by back-arc basin-like basalts, gabbros and dolerite/diorite dykes intruded into the Loma Caribe peridotite, as well as the Peralvillo Sur Formation basalts, capped by tuffs, shales and Campanian cherts. This dismembered ophiolitic stratigraphy indicates that the Bonao block is a fragment of an ensimatic back-arc basin. In the Jicomé and Jarabacoa blocks, the mainly Campanian group IV basalts of the Peña Blanca, Siete Cabezas and Pelona–Pico Duarte Formation, represent the subsequent stage of back-arc spreading and off-axis non-arc-like magmatism, caused by migration of the arc toward the northeast. These basalts have geochemical affinities with the mantle domain influenced by the Caribbean plume, suggesting that mantle was flowing toward the NE, beneath the extended Caribbean island-arc, in response to rollback of the subducting proto-Caribbean slab.  相似文献   

8.
The Emeishan continental flood basalt (ECFB) sequence in Dongchuan, SW China comprises a basal tephrite unit overlain by an upper tholeiitic basalt unit. The upper basalts have high TiO2 contents (3.2–5.2 wt.%), relatively high rare-earth element (REE) concentrations (40 to 60 ppm La, 12.5 to 16.5 ppm Sm, and 3 to 4 ppm Yb), moderate Zr/Nb and Nb/La ratios (9.3–10.2 and 0.6–0.9, respectively) and relatively high Nd (t) values, ranging from − 0.94 to 2.3, and are comparable to the high-Ti ECFB elsewhere. The tephrites have relatively high P2O5 (1.3–2.0 wt.%), low REE concentrations (e.g., 17 to 23 ppm La, 4 to 5.3 ppm Sm, and 2 to 3 ppm Yb), high Nb/La (2.0–3.9) ratios, low Zr/Nb ratios (2.3–4.2), and extremely low Nd (t) values (mostly ranging from − 10.6 to − 11.1). The distinct compositional differences between the tephrites and the overlying tholeiitic basalts cannot be explained by either fractional crystallization or crustal contamination of a common parental magma. The tholeiitic basalts formed by partial melting of the Emeishan plume head at a depth where garnet was stable, perhaps > 80 km. We propose that the tephrites were derived from magmas formed when the base of the previously metasomatized, volatile-mineral bearing subcontinental lithospheric mantle was heated by the upwelling mantle plume.  相似文献   

9.
The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceous–early Tertiary, along with pillowbasalts of the Caribbean–Colombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean 40Ar–39Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in Sr–Nd–Pb–Hf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 20–30% crystallization.The residual magmas from this fractionation contained  相似文献   

10.
The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.

Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (Nd (540 Ma)=−6.3 to −19.8; δ18O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (87Sr/86Srinitial=0.70794–0.70982). Two leucogranites and one aplite have higher initial 87Sr/86Sr ratios (0.70828–0.71559), but similar initial Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.  相似文献   


11.
A.S. Gaab  M. Jank  U. Poller  W. Todt 《Lithos》2006,87(3-4):261-275
Magmatic protoliths of Ordovician age have been identified in the metamorphic rocks of the Muráñ Gneiss Complex, Veporic Unit (Central Western Carpathians). Vapor digestion single zircon U–Pb dating yields an intrusion age of 464 ± 35 Ma (upper intercept) for the granite protolith. A lower intercept age of 88 ± 40 Ma records amphibolite-facies metamorphic overprint in the Cretaceous, during the Alpine orogeny. Geochemical and isotopic data suggest crustal origin of the orthogneiss. Ndinitial are between − 2.6 and − 5.0 and TDMNd between 1.3 and 1.5 Ga (two-step approach). 87Sr / 86Srinitial ratios vary between 0.7247 and 0.7120, and a steep REE pattern further constrains the crustal affinity of these rocks. Associated amphibolite bodies have Ndinitial values of 6.5, 87Sr / 86Srinitial ratio of 0.7017, and a flat REE pattern. They are interpreted as MORB derived metabasites. Whole-rock Pb isotope analyses define a linear array in a 206Pb / 204Pb vs. 207Pb / 204Pb diagram with an age of ca. 134 Ma, consistent with intense Alpine metamorphism and deformation.

These basement rocks of the Central Western Carpathians are interpreted as Ordovician magmatic rocks intruded at an active margin of Gondwana. They represent the eastern prolongation of Cambro–Ordovician units of the European Variscides, which were part of the peri-Gondwana superterrane and accreted to Laurussia during the Variscan orogeny. Variscan metamorphic overprint is not recorded by the isotopic data of the Muráñ Gneiss Complex. Alpine metamorphism is the most dominant overprint.  相似文献   


12.
Paleogene volcanic rocks crop out in three sedimentary basins, namely, Sanshui, Heyuan and Lienping, in the attenuated continental margin of south China. Lavas from the Sanshui basin which erupted during 64-43 Ma are bimodal, consisting of intraplate tholeiitic basalt and trachyte/rhyolite associations. Similar to Cretaceous A-type granites from the nearby region, the felsic member shows peralkaline nature [Na2O + K2O ≈ 10–12%; (Na + K)/Al≈ 0.98−1.08], general enrichment in the incompatible trace elements and significant depletion in Ba, Sr, Eu, P and Ti. Although both types of the Sanshui lavas have rather uniform Nd isotope compositions [Nd(T) ≈ +6 to +4]that are comparable to Late Cenozoic basalts around the South China Sea, the felsic rocks possess apparently higher initial Sr isotope ratios (ISr up to 0.713) and form a horizontal array to the right in the Nd vs. Sr isotope plot. Closed system differentiation of mantle-derived magmas in a ‘double diffusive’ magma chamber is considered for the bimodal volcanism, in which the trachytes and rhyolites represent A-type melts after extensive crystal fractionation in the upper portion of the chamber. Such A-type melts were later contaminated by small amounts (1–3%) of upper crustal materials during ascent. On the other hand, composition of lavas in the other two basins varies from tholeiitic basalt to andesite. Their Sr and Nd isotope ratios [ISr ≈ 0.705 to 0.711; Nd(T) ≈ +1 to − 5] and generally correlative Nb-Ta depletions suggest a distinct magma chamber process involving fractional crystallization concomitant with assimilation of the country rock. We conclude that these Paleogene volcanic activities resulted from the lithospheric extension in south China that migrated southwards and eventually led to opening of the South China Sea during 30-16 Ma.  相似文献   

13.
The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at 507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton (t=128 Ma) with a total area of 1200 km2, composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO2=69.85–72.35%, K2O/Na2O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb)N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are 206Pb/204Pb=16.707–17.055, 207Pb/204Pb=15.239–15.326 and 208Pb/204Pb=37.587–37.853, which are similar to that of the continental lower crust. Their Nd(t) values range from −16 to −20, and depleted-mantle Nd model ages (TDM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan.  相似文献   

14.
The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial 87Sr/86Sr ratios (0.707–0.708) and inhomogeneous negative Nd(T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ18O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial 87Sr/86Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget.

Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level.  相似文献   


15.
全球岛弧火山岩的铂族元素数据非常少。本研究对新疆阿尔泰山南缘中泥盆世北塔山组地层中的苦橄岩的铂族元素进行了分析,结果表明:其铂族元素的含量总体变化不大,其原始地幔标准化曲线总体上和已经报道的大西洋Lesser Antilles岛弧的G renada苦橄岩相似,均以富集钯组铂族元素(PPGE)为特征,但总体含量比后者高,而比大火成岩省的苦橄岩低。一些铂族元素的特征比值指示了苦橄岩和富辉玄武岩起源于软流圈地幔,而高的w(Os)/w(Ir)比值则暗示了源区中有深海沉积物的加入。w(Pd)/w(Ir)-w(N i)/w(Cu)图解说明本区的苦橄岩不是原始岩浆,而是原始的高镁玄武岩中有过剩橄榄石的加入,而w(Pd)-w(Cu)图解则说明原始岩浆硫不饱和。IPGE在俯冲带流体中相对稳定,而PPGE在俯冲带流体中的活动性相对较高。  相似文献   

16.
The island of Curaçao in the southern Caribbean Sea is composed mainly of a thick sequence (>5?km) of pillow lavas, grading upwards from picrites at the base of the exposed section, to basalts nearer the top. Modelling suggests that picrites are related to the basalts by fractional crystallisation. Initial radiogenic isotope ratios of the picrites have a restricted compositional range: ?Nd=+6.1 to +6.6, 87Sr/86Sr=0.70296–0.70319; whereas the basalts display a wider range of compositions: ?Nd=+6.6 to +7.6, 87Sr/86Sr=0.70321–0.70671. This variation in isotope ratios between basalts and picrites may be due to the assimilation of altered oceanic crust (or possibly partial melts of such crust) by a picritic magma along with fractional crystallisation. The relatively narrow range of Nd and Pb isotopic compositions in the Curaçao lavas suggests either that the source region was homogeneous, or that melts from a heterogeneous mantle source were well mixed before eruption. Chondritic to slightly light rare earth element enriched patterns, combined with long-term light rare earth element depletion (positive ?Nd), suggest that the lavas were formed by polybaric melting of spinel lherzolite, with small a contribution from garnet lherzolite melts. High-MgO lavas, the absence of a subduction related chemistry, and the chemical similarity to other oceanic plateaux, suggest a mantle plume origin for the Curaçao lava succession. The Curaçao volcanic sequence is part of an oceanic plateau formed at about 88–90?Ma, fragments of which are dispersed around the Caribbean as well as being obducted onto the western margin of Colombia and Ecuador. The occurrence of high-Mg lavas throughout this Cretaceous Caribbean–Colombian igneous province requires anomalously hot mantle (>200°?C hotter than ambient upper mantle) over a large part of a putative plume head, which is inconsistent with some mantle plume models.  相似文献   

17.
Several Paleozoic sutures in Southwestern China provide a record of the history of the Paleo-Tethys Ocean, whose birth and final closure are associated with the breakup and assembly of Gondwanaland. Recent studies indicate that there are widespread OIB-type mafic volcanic rocks within these suture zones and intervening terranes. This paper examines the geology and geochemistry of volcanic rocks in the Xiaruo-Tuoding area, a remnant passive margin succession of the Jinshajiang Paleo-Tethyan Ocean. The sedimentary and volcanic stratigraphy of this area is interpreted as a seaward dipping margin with a few continentward dipping normal faults. The available geochemistry of these volcanic rocks suggest that they are OIB-like basalts, characterised by SiO2 = 42.78–50.46 wt.%, high TiO2 contents (TiO2 = 2.2–3.55 wt.%), moderate MgO = 4.15–6.49 wt.%, Mg# = 0.37–0.50, high Ti/Y ratios (mostly > 450), large ion lithosphere elements enrichment, high strength field elements and rare earth elements, with La/Nb = 1.04–1.39, Ce/Yb = 18.38–30, Sm/Yb = 2.16–3.52, (87Sr/86Sr)i = 0.705350–0.707867, and Nd(t) = − 1.43–1.90. These geochemical and isotopic signatures are generally similar to those of the Emeishan flood basalts, which together with stratigraphic constraints, demonstrate that these volcanics were formed in a volcanic rifted margin, probably associated with a mantle plume. A new model is proposed to interpret the evolution of the Jinshajiang Paleo-Tethyan Ocean and its possible relationship to the Emeishan mantle plume. In this model, we argue that the opening of the Jinshajiang Paleo-Tethyan Ocean in the Carboniferous was caused by a mantle plume. The mantle plume was active to the east along the western margin of the Yangtze Craton between 300 and 260 Ma, from which the voluminous Emeishan flood basalts were erupted at 260 Ma. The closure of the Jinshajiang Ocean occurred since the Middle Permian. Continuous westward subduction generated the Jiangda-Weixi magmatic arc to the west of the Jinshajiang suture. This subduction also partly destroyed and/or tectonically sliced the volcanic rifted margin. Some seaward dipping volcanic-sedimentary sequences on the east flank of the Jinshajiang Ocean were preserved, but are strongly deformed.  相似文献   

18.
The Makran accretionary prism in southeastern Iran contains extensive Mesozoic zones of melange and large intact ophiolites, representing remnants of the Tethys oceanic crust that was subducted beneath Eurasia. To the north of the Makran accretionary prism lies the Jaz Murian depression which is a subduction-related back-arc basin. The Band-e-Zeyarat/Dar Anar ophiolite is one of the ophiolite complexes; it is located on the west side of the Makran accretionary prism and Jaz Murian depression, and is bounded by two major fault systems. The principal rock units of this complex are a gabbro sequence which includes low- and high-level gabbros, an extensive sheeted diabase dike sequence, late intrusive rocks which consist largely of trondhjemites and diorites, and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks, including radiolarian chert. Chondrite- and primitive-mantle-normalized incompatible trace element data and age-corrected Nd, Pb, and Sr isotopic data indicate that the Band-e-Zeyarat/Dar Anar ophiolite was derived from a midocean ridge basalt-like mantle source. The isotopic data also reveal that the source for basalts was Indian-Ocean-type mantle. Based on the rare earth element (REE) data and small isotopic range, all the rocks from the Band-e-Zeyarat/Dar Anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average E-MORB; fractionation was controlled by the removal of clinopyroxene, hornblende and plagioclase. Three 40Ar–39Ar plateau ages of 140.7±2.2, 142.9±3.5 and 141.7±1.0 Ma, and five previously published K–Ar ages ranging from 121±4 to 146±5 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic–Early Cretaceous. Plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the Masirah ophiolite which has crystallization age of (150 Ma). Like Masirah, the rocks from the Band-e-Zeyarat/Dar Anar ophiolite complex represent southern Tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90–100 Ma Bitlis-Zagros ophiolites (including the Samail ophiolite).  相似文献   

19.
Plate reconstructions at 30 Ma place northeastern Brazil over the Fernando de Noronha hotspot, presently 250 km offshore northeastern Brazil. Tertiary basaltic centers in northeastern Brazil may therefore be products of the Fernando de Noronha hotspot. We examined alkalic basalt from seven 30–13 Ma old centers in Rio Grande do Norte and Pernambuco states to assess this possible link. Compositions are primitive, where MgO concentrations range from 12.5 to 17 wt.%, and SiO2 from 41 to 48 wt.%. Trace-element abundances and Sr, Nd, and Pb isotopic compositions compare well with those of ocean island basalt: =0.7038–0.7051, =0.51266–0.51280, and =18.52–19.35. There are correlations among SiO2-undersaturation, incompatible-element abundances, relative percentages of partial melting (based on La/Yb and La/Y ratios), and the degree of isotopic ‘enrichment' inherited from mantle sources. There is also a negative correlation for La/Nb (0.6–0.86) vs. Ba/Nb (6.6–10.6), where lower La/Nb samples represent larger percentages of melting of a source comparatively enriched in radiogenic Sr. We attribute these compositional relationships to the lavas representing mixing of melts mainly from asthenosphere of largely HIMU plus DM characterization, probably the Fernando de Noronha plume, with melts from subcontinental lithosphere that was isotopically closer to EM1. Isotopic and trace-element compositions of the northeastern Brazil basalts are generally similar to those of Fernando de Noronha lavas (12–2 Ma), and some minor trace-element differences observed (e.g., more Zr, Nb, and less Th compared to northeastern Brazil basalts) are probably due to heterogeneity within the asthenospheric plume and to melt contributions from delaminated subcontinental lithosphere that may underlie Fernando de Noronha.  相似文献   

20.
The Liuyuan mafic and ultramafic rocks are exposed in Southern Beishan, which is along the southern branch of the Central Asian Orogenic Belt (CAOB). Zircon SHRIMP U–Pb dating showed that Liuyuan gabbros intruded during the early Permian (~ 270–295 Ma) coeval with the basalts and the ultramafic rocks were emplaced at about 250 Ma. The basalts are within–plate tholeiites with slight enrichment in light rare earth elements (LREE) relative to heavy rare earths (HREE) and small negative anomalies of Nb and Ta. Gabbros including olivine gabbros, olivine gabbronorites and troctolites are grouped into two: the cumulate gabbros are depleted in LREE and show small negative Nb and Ta anomalies but distinct positive Sr and Eu anomalies; non–cumulate gabbros resemble tholeiitic basalts. Lamprophyres and cumulate ultramafic rocks are characterized by large enrichment of LREE relative to HREE with depletion in Nb and Ta. The enriched Sr–Nd isotopic trend from DM towards the EM II end member component implies that the lithospheric mantle was progressively enriched with depth by the involvement of subducted crustal material due to the delamination of thickened mantle lithosphere after collision. The digestion of subducted crustal material into the mantle resulting in the metasomatized and enriched mantle is inferred to be an important process during crust–mantle interaction.  相似文献   

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